An intensive area of research is the synthesis of superhard nanocomposites (H ≥ 40 GPa) for use as wear-resistant coatings on tools and mechanical components as well as scratch-resistant coatings on optics. In particular, the pseudobinary TiN-SiNx system has become an archetype in the quest for such materials. The widely spread model for the phase structure of TiN-SiNx is that it consists of a set of nanometer-sized TiN crystallites encapsulated in a fully percolated SiNx "tissue phase" (1-2 monolayers thick) that is assumed to be amorphous.
In an attempt to gain better understanding of the origin of TiN-SiNx superhardness, we use in-situ STM and LEED to investigate the atomic-scale structure of the SiNx/TiN interface, of which very little is known. Samples were prepared in a multi-chamber UHV (base pressure < 10-10 Torr) system by reactive magnetron sputtering of epitaxial TiN layers onto single-crystal TiN(001) or TiN(111) substrates at temperatures between 700 and 900 °C followed by formation of SiNx overlayers at temperatures ranging from 600 to 800 °C.
We show both topographic (STM) and diffraction (LEED) evidence that (a) SiNx overlayers on TiN are crystalline with reconstructions including 2x2, c-3x3, and 1x5, depending upon SiNx coverage, surface orientation, and annealing temperature; and (b) TiN grows epitaxially on top of the SiNx layers. Specifically, our results show that for SiNx coverages near 1 ML, where maximum TiN-SiNx hardness is attained in both the nanocomposite and nanolaminate structures, the SiNx layer is not amorphous in the as deposited state.
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